Fundal height measuring small ):

So I went for my 30 week doctors appointment this morning and they measured my fundal height. It was measuring at 27/28 weeks. At my 28 week appointment it was measuring at 27 weeks, meaning my uterus pretty much has not grown in two weeks. I am so upset and am freaking out. My baby is moving around I feel him all the time and his heartbeat is 145. They scheduled an ultrasound for Monday but idk if I can wait that long the anxiety is killing me. 😥 has this happened to anyone ? My weight gain has been sufficient but I feel like my belly isn't growing ): what is going on ? 😥